Fuel alternatives to diesel, including gaseous fuels such as natural gas, hydrogen, ethane, and blends of such gaseous fuels, when used to power internal combustion engines, can significantly reduce emissions of pollutants when compared to the same engines fuelled by diesel. Emissions of pollutants such as nitrogen oxides (NOx), particulate matter (PM) and carbon dioxide (CO2) can be reduced when gaseous fuels are used to fuel diesel engines. The amount of such reductions depends on the fuel chosen amongst other variables. Further, diesel engines can be fuelled by gaseous fuel and still maintain the performance found when diesel fuel is used. For example, gaseous fuel directly injected under high pressure into the combustion chamber of a diesel engine can deliver performance that matches its diesel-fuelled counterpart.
Many gaseous fuels and some non-diesel liquid fuels are less auto-ignitable than diesel fuel. As such, conditions within the cylinder may not provide the necessary environment for auto-ignition when the same environment can auto-ignite diesel. Therefore, a method of assisting ignition is needed to ensure the gaseous fuel is ignited.
Injecting a pilot fuel is one method of initiating ignition of a directly injected quantity of gaseous fuel such as natural gas. Typically, a small amount of diesel fuel is added to a compressed intake charge within the combustion chamber when the piston is near top dead center. The diesel fuel auto-ignites. A main quantity of gaseous fuel is then injected. This quantity is ignited by the burning quantity of pilot fuel.
This method requires a source of a second, more auto-ignitable fuel. Therefore, such a system requires separate pilot fuel storage, and specialized fuel injection valve design and piping to manage pilot diesel fuel flow in addition to main fuel flow. Burning diesel fuel in the combustion chamber generates diesel emissions resulting in higher NOx, CO2 and PM levels than would otherwise be present if no pilot diesel fuel is employed.
Alternatively, hot surface or glow plug ignition can be employed to initiate ignition of a gaseous fuel that is directly injected into a compressed intake charge. Here, in general, a glow plug protrudes from the fire deck, a small distance into the combustion chamber. One of several fuel jets injected into the combustion chamber can be directed at the glow plug to initiate combustion of this fuel jet. In turn, this fuel jet, once ignited, will propagate a flame through the combustion chamber in an effort to ignite the other fuel jets.
One of the problems with this method is that the glow plug is a point source of heat within the combustion chamber. Therefore, it takes time for a first ignited fuel jet to propagate a flame throughout the combustion chamber. Additional turbulence may help but can be imperfect. Gas furthest removed from the glow plug may not ignite or may not burn incompletely. As such, the engine may run less efficiently, delivering less power because of incomplete combustion. Also, a higher percentage of unburned fuel can result in increased hydrocarbon (HC) emissions, see: Mueller, C. J. and Musculus, M. P., “Glow Plug Assisted Ignition and Combustion of Methanol in an Optical DI Diesel Engine”, SAE paper 2001-01-2004.
Spark ignition can also be used, however, like glow plugs, spark ignition ignites a flame at one point within the combustion chamber that propagates throughout the chamber. Therefore, combustion can be incomplete with this method too.
There is a need to address the problems noted above to improve engine efficiency and to reduce engine emissions.